This invention relates to the preparation of poly(aryl ether ketones). The term poly(aryl ether ketone) refers to polymers having the repeat unit of the formula EQU --CO--Ar--CO--Ar'--
where Ar and Ar' are aromatic moieties at least one of which contains a diaryl ether linkage forming part of the polymer backbone and wherein Ar and Ar' are covalently linked to the carbonyl groups through aromatic carbon atoms.
Preferably, Ar and Ar' are independently selected from substituted and unsubstituted phenylene and substituted and unsubstituted polynuclear aromatic moieties. The term "polynuclear aromatic moieties" is used to mean aromatic moieties containing at least two aromatic rings. The rings can be fused, joined by a direct bond or by a linking group. Such linking groups include for example, carbonyl, ether, sulfone, sulfide, amide, imide, phenoxy, azo, alkylene, perfluoroalkylene, and the like. As mentioned above, at least one of the Ar and Ar' contains a diaryl ether linkage.
The phenylene and polynuclear aromatic moieties can contain substituents on the aromatic rings. These substituents should not inhibit or otherwise interfere with the polymerization reaction to any significant extent. Such substituents include, for example, phenyl, cyano, alkyl, 2-alkynyl, and the like.
Poly(aryl ether ketones) having the following repeat units (the simplest repeat unit being designated for a given polymer) are preferred: EQU --Ph--O--Ph--CO-- EQU --Ph--O--Ph--CO--Ph--CO-- EQU --Ph--O--Ph--O--Ph--CO-- EQU --Ph--O--Ph--O--Ph--CO--Ph--CO-- EQU --Ph--O--Ph--CO--Ph--O--Ph--CO--PH--CO-- EQU --Ph--O--Ph--O--Ph--CO--Ph--O--Ph--CO--
where Ph denotes p-phenylene.
Poly(aryl ether ketones) possess many desirable properties, among which are their high temperature stability, their mechanical strength, and their resistance towards common solvents. In general, polymers in which the valence positions of the aromatic rings in the repeat unit are para with respect to each other are preferred. Such polymers have greater chemical, thermooxidative, and solvent resistance than those in which the valence positions are meta or ortho.
Poly(aryl ether ketones) generally can be prepared by either one of two methods: (1) electrophilic synthesis, in which an aryl ketone linkage is formed during the polymerization step, or (2) nucleophilic synthesis, in which an aryl ether linkage is formed during the polymerization step.
In the polymerization step of an electrophilic synthesis a carboxylic acid derivative, usually an acid halide, reacts with a phenoxy group to form a diaryl ketone. Electrophilic syntheses suffer from certain disadvantages, among which are the requirement for hydrolytically labile acid halide monomers and, in some but not all procedural variants, the use of corrosive media such as hydrogen fluoride. See, for example, U.S. Pat. Nos. 3,441,538 to Bonner, 3,441,538 to Marks and 3,953,400 to Dahl.
In the polymerization step of a nucleophilic synthesis a phenoxide salt reacts with an aryl halide to form a diaryl ether. The halide thus displaced must be activated towards nucleophilic displacement by an electron withdrawing substituent ortho or para to it. The present invention concerns nucleophilic synthesis.
A recurring limitation in nucleophilic syntheses of poly(aryl ether ketones) is the paucity of suitable solvents for the polymerization. The solvent should have sufficient solvent power to keep the growing polymer chain in solution, preventing its premature precipitation before the attainment of high molecular weights. It should be high boiling, enabling polymerization to occur at the higher temperatures at which the reaction proceeds faster and the solubility of the polymer is greater. Naturally, the solvent should be inert under the reaction conditions.
Solvents disclosed in the prior art have generally been drawn from the group broadly designated as dipolar aprotic solvents. See, for example, U.S. Pat. No. 3,764,583 to Newton et al. Among these, the ones most commonly employed have been the sulfones and sulfoxides, in particular the diaryl sulfones. See, for example, U.S. Pat. No. 4,108,837, to Johnson et al., and U.S. Pat. No. 4,320,224 to Rose et al. The number of sulfones or sulfoxides that have actually been used is quite limited, consisting primarily of dimethyl sulfoxide, dimethyl sulfone, tetramethylene sulfone (sulfolane), diphenyl sulfoxide, and diphenyl sulfone. The alkyl sulfones and sulfoxides have the drawbacks of possessing acidic alpha-hydrogens and boiling at relatively low temperatures, while their aromatic counterparts are not readily available and are often expensive. Therefore, it is desirable to develop alternative synthetic methods for poly(aryl ether ketones) and, particularly where a nucleophilic synthesis is contemplated, alternative solvent systems.